Saccharomyces cerevisiae can utilize urea as a sole nitrogen source by virtue of its ability to degrade the compound to two moles of ammonia and two moles of CO2. We have shown that this conversion is accomplished by two enzymes acting in concert; a urea carboxylase catalyzing reaction (1): Urea plus ATP plus HCO minus 3 yields Allophanate plus ADP plus Pi and an allophanate hydrolase catalyzing reaction (2) Allophanate yields 2 NH3 plus 2 "CO2". Mutants defective in each of these enzymes have been isolated and a cross of the two types of mutants yielded no recombinants suggesting that the pertinent structural genes are linked. In view of these results, we are studying the following areas: (1) the characteristics of urea carboxylase with regard to its mechanism of action and subunit structure, (2) the characterization of allophanate hydrolase with regard to its mechanism of action and subunit structure, (3) the details surrounding the assembly of the subunits of urea carboxylase, (4) the details surrounding the assembly and interaction of urea carboxylase and allophanate hydrolase in a multi-enzyme complex, (5) a thorough study of the linkage relationship between the genes coding for urea carboxylase and allophanate hydrolase to determine if they are contiguous (this will be done by isolating independent nonsense mutants with the expectation that some will be pleiotrophic and also by monitoring the frequency of single and double site gene conversion at the two loci), and (6) characterization of the physiological control of this system by ammonia, arginine, purine degradation and urea.